Freshwater Communities & Lentic Waters

- Freshwater Ecology

Previous pages elaborated on the importance of light to the freshwater community. Some factors can affect the amount of light received by autotrophic organisms (organisms that perform photosynthesis) can affect their level of photosynthesis and respiration, hence affect their abundance and therefore and subsequent species that rely on them.

Man has continuously polluted water sources, especially since the industrial revolution. Litter for example, and especially non-biodegradable litter, will block out light for light dependant organisms. An oil spillage will also have the same effect, perhaps more extreme as the oil will situate itself on the surface of the water and block out light.

Organic material and sediment can enter the still water environment via dead organisms in the area, and water flowing into the area from hills and streams. Buoyant material will also block out light required by the primary producers of the ecosystem.

When water moves, the friction caused by the moving water against the water bed and its banks will result in disturbing loose sediment. Depending on the weight of this sediment, heavier particles will slowly sink back to the bottom of the body of water while lighter materials will remain suspended in the water. The lightest material will rise to the surface, resulting in less light available to organisms underneath the surface.

Naturally, the consequences of the above will result in less light for organisms that rely on photosynthesis as a means of food, and subsequently means that organisms that feed on these autotrophic organisms will soon find that their food source is less freely available.

Another major factor affecting still water communities is the oxygen concentration of the surrounding area. Oxygen concentration is primarily affected by three factors

The surface area which the body of water is exposed to the open air environment

The circulation of water, chiefly due to temperature differentiations in different areas of the water body (convection currents)

Oxygen created as a result of respiration, consumption, and the oxygen consumed by animals and bacteria.

As mentioned on previous pages, temperature can also affect the concentration of oxygen available, which in turn, means that the depth of the water will therefore also have an effect. In turn, carbon dioxide levels, which are closely related to the oxygen levels available will be required by organisms undergoing photosynthesis. The availability of these will affect the organisms in the ecosystem. Their relationships with temperature will also affect their availability.

Evidently, some of these factors vary through different conditions, and changes in one of the factors usually results in changes with the others. This is also the case of pH, for example, as an increase in carbon dioxide results in a drop of pH.

With this information, and the previous pages in hand, we can now understand how organisms survive in these habitats in relation to these conditions described.